234 



CHAPTER 17 



1 2 



13 3 4 



r? 



3 3 



I 



5 5 



FIGURE 17-10. Heterozygous reciprocal trans- 

 locations and circle formation. Chromatids 



arc not shown. 



OgOUS chromosomes, the ends of dilTerent 

 chromosomes in a genome are given dif- 

 ferent numbers. Suppose, at some time in 

 the past, a dicentric reciprocal translocation 

 occurred between the tips marked 2 and 3 

 (Figure 17-10A, B). This rearrangement 

 in heterozygous condition ( B ) would pro- 

 duce an X-shapcd configuration at the time 

 of synapsis in prophase I (C) and a circular 

 appearance at metaphase I-early anaphase 

 1 (D). In this way a circle of four chro- 

 mosomes would be produced. 



If a second reciprocal translocation oc- 

 curs between any chromosome arm in a cir- 

 cle of four and an arm of some other pair 

 of chromosomes, a circle of six chromosomes 

 will form in the individual heterozygous for 

 both reciprocal translocations. This type of 

 formation is illustrated in Figure 17-10D, 

 E; D shows the configuration before arms 

 4 and 5 have exchanged, E shows the circle 

 of six produced in meiosis after this ex- 

 change. Still larger circles can be formed 

 by successive interchanges of this type; six 

 such interchanges are required to form a 

 circle of 14 chromosomes. The presence 

 of reciprocal translocations in heterozygous 

 condition could explain how various sized 

 circles containing even numbers of chromo- 

 somes are produced in Oenothera. 



Although the cytogenetic analysis of Oeno- 

 thera is known in some detail, the picture 

 is, however, not yet complete. One of the 

 questions remaining is: What is the mech- 

 anism whereby alternate chromosomes in a 

 circle proceed to the same pole during meio- 

 sis? No fully acceptable answer to this ques- 

 tion has yet been given. A second question 

 stems from the fact that almost all the dilTer- 

 ent strains or races of Oenothera found in 

 nature form a circle of 14. Are the six 

 translocations involved the same in all races? 

 No — for if they were, viable hybrids be- 

 tween races would form either circles of 14 

 or seven separate chromosome pairs at meio- 



